Field
This invention relates generally to a Z-pin for mechanically locking laminate layers in a composite structure and, more particularly, to a Z-pin including two opposing bi-metal structures selectively secured together and inserted into a composite laminate structure, where curing the composite structure causes the Z-pin to bend as a result of the difference in the coefficient of thermal expansion between metal strips in the structures so as to mechanically lock laminate layers in the composite structure.
Discussion
Many structures, such as certain aircraft structures, certain high performance vehicle skins, etc., are composite laminate structures that include a plurality of laminate layers, such as fiber glass layers, fiber reinforced plastic layers, fiber carbon layers, etc. For example, some aircraft skin structures include thirty or so laminate layers each having a thickness of about 0.001-0.015 inches. Typically, these composite layers are formed by laying down an interwoven pattern of fibers, such as carbon fibers, that are immersed in a liquid resin, where the resin is cured by heating, which causes it to harden. The several layers are bonded or secured together by a suitable bonding technique, such as co-curing, adhesive bonding, etc.
The orientation of the fibers in the laminate layers of these types of composite structures typically has high strength in the x and y direction along the length of the fiber, but has a relatively low strength in the z-direction across the fibers. Therefore, it is known in the art to provide mechanical fastening devices that are inserted across the layers to provide increased strength in the z-direction. One well known technique is referred to as Z-pinning that employs Z-pins inserted into and across composite laminate layers in a z-direction to improve resistance to delamination, increase out of plane shear, and increase damage tolerance by providing reinforcement in the z-direction of the structure and not relying simply on adhesive bonding.
A typical Z-pin will be quite small in diameter, such as 0.010-0.020 inches, where a large number of the Z-pins, for example, 60-100, may be inserted cross-wise into the laminate structure per square inch. In one insertion technique, the Z-pins are partially inserted into a top surface of one of the laminate layers while the laminate layers are in a partially cured or pre-preg state, where the resin is still soft and pliable. An ultrasonic tool is positioned against a group of the Z-pins, where the ultrasonic energy creates some level of heating that further softens the resin and allows the Z-pins to be inserted through the laminate layers without interfering with the fibers.
A traditional Z-pin has a cylindrical shape. However, other Z-pins come in variety of shapes and sizes. U.S. Pat. No. 6,514,593 issued to Jones et al., titled Mechanically Locking Z-Pins, assigned to the assignee of this application and herein incorporated by reference, discusses disadvantages of the traditional Z-pin and proposes shaped Z-pins having increased Z-pinning in the z-direction. Shaped Z-pins typically provide superior performance to traditional cylindrical Z-pins because they reduce pullout from the composite matrix by increasing surface area for adhesive bonding, mechanically locking into the matrix, and locking into the fiber reinforcement. However, because of the shape of these types of Z-pins, they are more difficult to insert into the laminate structure using the ultrasonic tool while the laminate structure is in the pre-preg state because the shape of the Z-pin alters the position of the fibers in the composite layers as they are being inserted. Often, this type of damage to the fibers during insertion of the shaped Z-pins affects the structural integrity of the layer.